PROJECT SUMMARY/ABSTRACT This is a K08 Mentored Clinical Scientist Research Career Development Award application for Dr. Jeannette S. Messer, a Pathway to Independence Instructor in The University of Chicago Department of Medicine. Her long-term goal is to become an internationally recognized expert and researcher in the field of host-microbe interactions in human inflammatory diseases. In order to achieve this goal, she will require training to address four defined gaps in her knowledge: translational research [Gap #1], gnotobiotic mouse models [Gap #2], microbiology [Gap #3], and bioinformatic techniques to study microbial populations [Gap #4]. Dr. Messer has assembled a multidisciplinary team of expert mentors and scientists with significant experience in human inflammatory bowel disease, gnotobiotic mouse utilization and management, microbiology, and bioinformatic approaches to analysis of microbial populations of the gut to close these gaps in knowledge and remove the identified barriers to her transition to independence and achievement of her long term goal. Inflammatory bowel diseases (IBD) are diseases of chronic diarrhea and abdominal pain that affect an estimated 1.6 million Americans. Bacteria are thought to play an important role in IBD pathophysiology through activation of mucosal immune responses. However, the mechanisms through which bacteria trigger or perpetuate mucosal immune responses are still poorly understood. Therefore, there is an urgent unmet need to determine how bacteria contribute to immune activation and mucosal damage during IBD in order to develop effective therapies that take the microbial component of the disease into account. High mobility group box 1 (HMGB1) is produced in intestinal epithelial cells (IEC) in response microbes. HMGB1 is decreased in IEC from active IBD lesions and conditional deletion of HMGB1 from IEC exacerbates colitis in murine models. The colitis seen in mice lacking IEC HMGB1 is characterized by increased IEC death and in vitro studies have shown that IEC lacking HMGB1 are more susceptible to apoptosis when challenged with the bacterial cell wall component muramyl dipeptide. HMGB1 is also found in stool and concentrations of this protein in the stool are increased during active IBD. Dr. Messer has now found that mice conditionally deficient in IEC HMGB1 also have evidence of dysbiosis and the intestinal barrier between bacteria and host tissues is compromised. These factors suggest that HMGB1 may have a role in antimicrobial defense in the intestine. Therefore, experiments in this study will test the hypothesis that HMGB1 plays an essential role in maintaining gut microbial commensalism and loss of HMGB1-mediated gut antimicrobial defense, in conditions such as IBD, leads to microbe-induced inflammation and mucosal damage. In Aim 1 Dr. Messer will determine mechanisms of HMGB1-mediated gut antimicrobial defense with the goal of understanding how HMGB1 regulates bacterial colonization and virulence. Aim 2 will determine mechanisms whereby derangement of HMGB1-mediated gut antimicrobial defense contributes to mucosal damage and IBD. The goal of this aim is to determine the impact of HMGB1 on host responses to gut bacteria. In each of these aims she will utilize innovative and cutting-edge techniques including germ-free mice, primary intestinal epithelial cell cultures, and bacterial metagenomics. She will also utilize human samples to investigate HMGB1 expression and changes to the types or behavior of bacteria colonizing the intestinal mucosa during IBD. Execution of these aims will provide didactic and hands-on training that will address the identified deficiencies in Dr. Messer's knowledge base and allow her to advance in her career path toward becoming an independent investigator. Furthermore, understanding HMGB1-regulated host-microbe interactions will directly address an urgent, unmet need in the field of IBD pathophysiology and open up new avenues of investigation as well as suggest novel therapies for these diseases.